Handling of dusts, etc.



- July 11, W39, C sMn-H 2,165,946

HANDLING OF DUSTS, ETC

Filed July so, 1935 Patented July 11, 1939 UNITED STATES PATENT OFFICEHANDLING OF DUSTS, ETC.

poration of Delaware Application July 30, 1936, Serial No. 93,395

4 Claims.

This invention relates to the handling of dusts or fine powders such asfinely powdered pigments and other organic and inorganic products, andapparatus therefor. It includes particularly 5 the conveyance of suchdusts or powders through suitable ducts, the production of such dusts inpulverizers, their separation in devices such as cyclones and thescreening of such materials and other operations in which such materialsare handled in dust form. More particularly the invention relates to thehandling of such dusts which tend to adhere to metallic surfaces and,according to the improved method of this invention the dusts are handledin apparatus which has been lined with rubber or other material havingelectrical properties similar to rubber. It has been found that byrubber-lining ducts, pulverizers, separators, etc., and covering theframe work of screens with rubber the adherence of the dust to exposedsurfaces is very substantially reduced and may be almost entirelyeliminated, and the clogging of apparatus thus prevented. The inventionincludes both the method of handling dusts or powders, and apparatus forhandling the same.

It is not unusual to line ducts, pulverizers, etc. with rubber or likematerial to prevent abrasion of the exposed surfaces. The only effect ofsuch lining is to serve as a cushion to protect the metallic surfacefrom abrasion. Such linings are known in sand-blasting apparatus and thelike. In such operations there is no tendency for the sand (or otherparticles carried by the air blast) to adhere to the walls of theapparatus since the 1;.) abrasion of the walls keeps them clean.v

As distinguished from such prior art the present invention relates tothe transportation and other treatment of dust particles which tend toadhere to exposed surfaces of the treating apparatus. In the manufactureof pigments, dyestuffs, organic compounds of many types and manyinorganic materials, it is customary to convey the material in dust formfrom one location to another by blowing it through a metal duct. Thematerial may for example, be blown from a pulverizer to a separator orsifter through such a duct. The material is carried by a blast of airwhich is suflicient to effect the desired transportation withoutabrasion of exposed surfaces of the apparatus. It is generallyrecognized that in such operations the dust often tends to collect onthe walls of the apparatus and deposits are formed which eventually tendto clog the ducts or other apparatus. In pulverizers the dust particlesbuild up and clog the pulverizer so that it is necessary at notinfrequent intervals to cease all pulverizing operations in order thatthe deposit may be removed from the pulverizer. In ducts it is customaryfor employees to tap or pound the ducts to loosen any deposit and thusfree the ducts. The pounding or hammering to which workmen subject suchapparatus distorts its shape and thus actually increases the tendency ofdust particles to collect and necessitates replacements all toofrequently. Similarly in separators of the cyclone type, etc., cloggingdifficulties are quite common. In screen where balls, for example, areused to vibrate the screen the dust tends to become packed on the framesof the screen by the pounding action of the balls. It has been foundthat the tendency of dust particles to thus adhere to the walls of theducts or other apparatus can be overcome by lining the walls with rubberor a rubber-like material.

A very extensive investigation of this situation and particularly theadherence of dust particles to the walls of ducts, indicates that theadherence of the duct particles to the walls is in large part due to anelectrical phenomenon. Although the flexibility of rubber tends toprevent adherence and caking of the dust, this is a less importantfactor. The value of rubber in preventing the adherence of dustparticles to the exposed metallic surface appears to be due largely tothe unique electrical properties of the rubber and particularly to thefact that rubber is a dielectric, it is easily charged withtribe-electricity, and any static charge formed on it is not readilydissipated. Another property of rubber which makes it a desirablematerial is its low thermal conductivity. (In stating that rubber is agood dielectric it is appreciated that differently compounded rubbersvary in dielectrical properties and those exceptional rubbers which arespecially compounded so as to make them better conductors of electricityare not included among the rubbers referred to herein in general terms.)

It has long been recognized that dust particles settle out on coolsurfaces more rapidly than on heated surfaces. This is noticeable in aplastered house where the dust settles more rapidly on the plastersurfaces of the walls between the laths than it does upon the surfacesover the laths. The laths are poorer conductors of heat than the plasterand the portions of the walls between the laths therefore cool morerapidly than the portions over the laths and the dust consequentlyseparates out more readily on these cooled portions. This is, no doubt,due to the fact that on contacting with a cool surface the dustparticles lose some energy and fail to rebound, thus being deposited.The same law apparently covers the action of dust particles inrubber-lined metallic ducts, etc. At least a portion of the par ticleswhich do not have sufficient energy to rebound from a metallic pipe,have sufllcient energy to rebound from the rubber lining.

A more important result of rubber-lining the ducts is due to the factthat rubber is a dielectric and is easily given a tribe-electric charge.Most dust particles possess an electrostatic charge. Most pulverizingoperations produce such a charge on the pulverized materials. They alsoreceive a charge on being blown through a duct. Likewise the blowing ofair over the walls of a duct produces a tribe-electric charge on thewalls. If the walls are of metal this charge is of course readilydissipated. Rubber being a dielectric, the rubber lining of the ductsretains the charge. In a majority of cases the charge on dust particlesis negative, and the charge on vulcanized rubber compounded in the usualway is likewise negative. With like charges on the dust particles andthe walls of the ducts, the tendency is for the dust particles to berepelled from the walls and the advantages of using rubber lined ductsmay be largely attributed to this fact.

The charge on dust particles hasbeen extensively investigated. Incertain instances the sign of the charge was found to depend uponwhether or not the dust particles possessed a charge before being blownthru the rubber-lined duct. Regardless of the sign of the charge it wasfound that less dust adhered to the rubber lining than to the metalduct.

The invention has been studied more particularly in connection with thehandling of mercaptobenzothiazole (known in the trade as Captax). Inpulverizing this material in the ordinary rotary pulverizer it was foundthat the Captax adhered to the walls of the pulverizer and became packedon the walls so that it was necessary to open the pulverizer at frequentintervals to remove the deposit. On coating the walls of the pulverizerwith rubber such packing was negligible. From the pulverizer the Captaxwas blown to a cyclone separtor through metal ducts. It adhered to theducts and it was necessary at frequent intervals to pound the ducts orotherwise cause them to vibrate to separate adhering Captax particles.The dust collected in the ducts, at first, was a fuzz and this rapidlybuilt up into an appreciable deposit which eventually would havecompletely closed the ducts. In plant operations the ducts are tapped atintervals to loosen the dust adhering to them and thus prevent theirbecoming stopped. It was found that by rubber lining these ducts thedeposit of Captax upon them was almost entirely prevented. Likewise inthe cyclone separator used to separate the dust from. the current of airemployed to carry it along, the collection of dust on the metallic wallsof the cyclone can be prevented by rubber lining the cyclone. The Captaxrecovered in the cyclone was sifted in a sifter cf the gyratory type inwhich rubber balls in a compartment below the screen of the sifterbouncing against the wooden frame of the screen greatly expedite thesifting of the material. Past experience had shown that the Captax dustcollected on the frames of the screens and that the rubber ballsbouncing against this packed it intoa. cake which later flaked ofi andreduced the average fineness of the I sifted particles. The collectionof Captax on the screen frames was eliminated by covering the frameswith rubber.

In all of these experiments rubber having the following approximateformula was found satisfactory:

Per cent Rubber 48. 7 Black 43. 5 Zinc oxide 2. 9 Sulphur Accelerator 4.9 Softeners Rubber of this type will give satisfactory results when usedfor lining equipment of the type shown in the drawings. Other rubbersdifferently compounded will likewise be satisfactory, for example:

The invention will be described in greater detail in connection with theaccompanying drawing but it is to be understood that it is not limitedto the process and apparatus there described.

Fig. 1 shows apparatus for pulverizing, separating and screening, etc.Fig. 2 is a plan view of the screen;

Figs. 3 and 4 are sectional views of the screen on the lines 3-3 and4-4; and

Fig. 5 is an enlarged view of the pulverizer with one side open to showthe interior.

In handling mercaptobenzothiazole in'such ap paratus it is fed throughthe chute I into the pulverizer 2 where it is broken down to smallparticles which are drawn into the separator 3 where the coarserparticles are separated from the finer particles. The coarser particlesare returned to the pulverizer and the finer particles are conveyedthrough the pipe 4 by the blower 5 through the pipe 6 into the cycloneseparator I. The finer particles separate from the air in this cycloneseparator and the air is returned through the pipe 8 and re-used toremove the pulverized material from the pulverizer 2.

From the cyclone separator I the finely divided mercaptobenzothiazole isdischarged through the spout 9 .onto the screen In. The screenedmaterial is collected through the opening Hand the coarser materialwhich does not pass through the screens is delivered through the openingl2. This coarser material is advantageously returned to.

the pulverizer.

A small quantity of fresh air is taken into the air-circulating systemthrough cracks and openings and this is desirable as it keeps the aircool. The excess air is vented through the pipe I3 and the cloth tubes I4. Superfine particles separated in the cloth tubes are collected in anysuitable manner from the bottom opening l5 below these cloth tubes.

, As indicated the enclosing walls of the air fiotation system arerubber lined. The interior of the pulverizer 2 is lined with rubber 20as shown in Fig. 5. The mercaptobenzothiazole in unpowdered form is. fedto the pulverizer through the opening 2| by means of the chute or pipeI. It is subdivided by contact with the blades or propellers 22 whichare rotated at high speed, e. g., 3500 R. P. M. This breaks andpulverizes the mercaptobenzothiazole. Hard unpulverizable material, suchas foreign matter, etc., is discharged through the kick-off 23. Airwhich enters the pulverizer through the pipe 8 carries the pulverizedmaterial up through the outer channel 25 of the separator 3 to the topof the separator where coarser material settles out from the finermaterial by gravity and falls back to the pulverizer. This outer channel25 is' rubber-lined as shown to prevent the adherence of the pulverlzedmaterial thereto. In the top of the separator the air and pulverizedmaterial passes through openings which connect the outside channel withthe interior portion of the separator. These openings are not shown.Their size is regulated in the usual manner by adjustable vanes whichlikewise are not shown. This type of apparatus (except for the rubberlining) is well known in the art'and other apparatus which accomplishesthe same purpose may be substituted for it and those parts of such otherapparatus through which the pulverized material is air-floated maylikewise be rubber-lined to advantage.

In the separator 3 the heavier particles of mercaptobenzothiazole settleout of the air and are returned to the pulverizer 2 and the lighterparticles pass through the rubber-lined pipe 4 into the blower 5 fromwhich they are blown through the pipe 6 to the separator I. The blower 5and pipe 6 are both rubber-lined. This prevents the accumulation ofmercaptobenzothiazole on the walls.

The separator I operates on the well-known cyclone principle. It is soconstructed and operated that the mercaptobenzothiazole particles settleout of the air and are collected through the spout 9. The air thenpasses through the pipe 8 back to the pulverizer. The separator I andpipe 8 are both rubber-lined to prevent the accumulation ofmercaptobenzothiazole on their walls. Excess air is conducted throughthe pipe 3 to the filter bags H. The pipe 13 is rubberlined and the top26 and the bottom 21 of the filter which are connected by the clothtubes H! are both rubber-lined to prevent the accumulation ofmercaptobenzothiazole on the walls.

The pulverizer shown is merely illustrative of many different types ofpulverizers and subdividers that may be employed. Ball mills, hammermills, roller mills, etc., which may be employed for subdivision of suchmaterials as mercaptobenzothiazole, may be rubber-lined to prevent theadherence of the mercaptobenzothiazole etc. to the walls thereof.Instead of employing a pressure system of air flotation a vacuum systemmay be utilized and the walls of the suction fan and treating apparatusmay berubber-linedtoprevent the adhesion of mercaptobenzothiazole orlike material thereto. Although in the preferred method and apparatusillustrated in the drawing, all of the walls with which the air andpulverized material come in contact are rubberlined, the invention isnot limited thereto, as rubber-lining may be omitted from those portionsof the walls where no difficulty is experienced with adherence andaccumulation of mercaptobenzothiazole. v

The finely divided material which separates in the cyclone I isdelivered onto the screen 33 of the screening apparatus Hi. Theapparatus shown is of the Rotex type. Theapparatus 30 imparts a circularmotion to the midpoint of the head 3| of the screen, and the corners ofthe foot 32 of the screen slide back and forth in suitable guidesprovided for the purpose. The screen 33 is supported by partitions 35which divide the area under the screen into many sections. Each sectioncontains several balls of resilient rubber. As shown in Figs. 3 and 4the partition running under the middle of the screen and the crosspartitions all have beveled edges so that as the screen is moved fromside to side and the balls are thrown against the partitions they bouncefrom the beveled edges against the screen which they support and thuscontinuously tap the screen on the under side and cause the finematerial to pass through it. The beveled edges of these partitions andthe edge of the frame 36 which supports the screen are rubber coated asshown in Figs. 3 and 4. Without such rubber coatingmercaptobenzothiazole stuck to these surfaces. By coating the surfaceswith rubber the adhesion of the mercaptobenzothiazole is prevented. Theadhesion of mercaptobenzothiazole to those surfaces was particularlyundesirable because with the constant bouncing of the balls onto thesesurfaces the mercaptobenzothiazole which stuck to them was packed into ahard cake which eventually became separated from the partitions asfairly large flakes as the operation continued and became mixed with thefinely screened material and caused difficulty.

The material which passes through the screen 33 readily passes throughthe screen 31 which is of large mesh and supports the balls 38. Belowthis coarser screen 31 is a collection compartment through which thefine material passes to the opening I I. The coarse material which doesnot pass through the line screen 33 passes over the foot of the screenand is collected through the opening H. The entire surface of thecollecting compartment located below the screen 31 is advantageouslycoated with rubber to prevent the adhesion of the screenedmercaptobenzothiazole thereto, and the end compartment through which thecoarser material passes to the opening I2 is advantageouslyrubber-lined.

Although but one particular type of screen is shown in the drawing, itis illustrative of the application of the invention to other screens ofthis general type. In general any surface under a screen which isexposed to screened material is advantageously covered with rubber. Thissurface may be a metal surface or a wooden surface. The screen may be ofstationary type and it may be of the type in which mechanical means isprovided for tapping or moving the screening cloth to aid the passage offine material therethrough. The screen may be of the type in which theframe and screen supported thereby are violently agitated to cause thefine particles to pass through the screen. In any of these types ofapparatus supporting frames, etc., under the screen and collectioncompartments, etc., are advantageously rubber covered to prevent theaccumulation of the screened material on them and thus to prevent theagglomeration of fine particles of the screened material to a size whichwould not pass through the screen. Contamination of the screenedmaterial by such agglomerates is thus prevented.

The screen may be of the type in which severa screening cloths arearranged one above another and in which the powdered material isprogressively separated in-to coarser and finer particles in passingthrough screens of different mesh. The walls enclosing screens of thistype are advantageously rubber covered to prevent the accumulationthereon of the material being screened. All types of vibrating screensare improved by the rubber covering of solid surfaces exposed to thefinely divided product.

In all these various applications of the invention the rubber may beapplied to the various surfaces in different ways. For example, rubbersheeting may be tacked or cemented or otherwise fastened to the walls.The rubber may be applied as a cement where this is satisfactory. Inapparatus, such as the screen shown in the drawing, the variouspartitions indicated may be rubber coated by slipping rubber tubing overthem. The rubber will ordinarily be vulcanized before being applied as aprotective surface, but this is not necessary as it may be vulcanized inplace. Instead of using pure rubber, rubberized fabric may be employedin suitable places as for lining pipes, etc. Both wooden and metal pipesand pipes of other materials may be rubber coated according to theinvention to prevent the adhesion of powdered material thereto.

Although in the above description the handling of Captax has beendescribed more particularly it is to be understood that other dustparticles may be similarly handled to advantage in rubber lined ducts,etc. For example such dusts as lithopone, zinc oxide, andtetramethylthiuramdisulphide (tuads), may advantageously be handled insuch equipment. Dusts which possess a positive charge are advantageouslyhandled in ducts lined with rubber which acquires a relatively positivetribe-electric charge. I

Instead of using natural rubbers, synthetic rubbers such as Duprene maybe employed. Rubber derivatives such as rubber hydrochloride have givensatisfactory results. rubber-like materials are good electricalinsulators, are easily given-a tribo-electric charge, and have atendency to retain any static charge produced in or on them.

Although the invention has been described more particularly inconnection with certain units of All of these various apparatus, it isto be understood that it applies more generally to the handling of dustsin rubber lined metal equipment to which the dust would adhere morereadily if the equipment were not rubber lined,

I claim:

1. The method of treating a material which on pulverizing possesses anegative triboelectric charge, which comprises dry pulverizing it todust form in a rubber-lined pulverizer, floating it in a blast of airthrough a rubber-lined duct to a rubber-lined separator and thereinseparating the dust from the air used to convey it.

2. The method of treating a non-corrosive material which on drypulverizing possesses a negative triboelectric charge, which comprisespulverizing dry the material to a dust in a rubberlined pulverizer,conveying the dust through a rubber-lined duct by flotation in a blastof air, separating the dust from the air and then screening the dustdown through a screen the frames of which are covered with rubber whileagitating the screen by bouncing rubber balls against the under surfaceof the screen and rubber-covered frames.

3. The method of pulverizing a material which in dust form has anelectric charge which on contact with the metal of the pulverizer isdissipated with deposition of dust on the pulverizer and eventual cakingof the dust in the pulverizer which comprises dry pulverizing saidmaterial to a dust in a rubber-lined pulverizer.

4. The method of dry pulverizing materials, which because of theirtriboelectric charge when pulverized adhere to metals but not to arubberlike dielectric, which comprises grinding said materials in ametal mill lined with said dielectric at those positions of the mill atwhich the dust would adhere if not lined.

CLAUDE H. SMITH.

